WASHINGTON -- Outgoing NASA administrator Michael Griffin warned Tuesday that the space agency may have to lay off an unspecified number of contractors on the back-to-the-moon Constellation program if Congress continues to freeze NASA spending below the $17.6 billion requested for this year by the Bush administration.

Griffin told reporters at a breakfast sponsored by the Space Foundation that continuing funding of the space agency at the level specified in a temporary budget resolution would force the personnel cutbacks.

Griffin, who said his resignation would take effect at noon on Jan. 20, declined to specify the number of contractors who might lose their jobs or where the cutbacks might take place.

Griffin said President-elect Obama faces four tough choices on the space program that must be made in the first months of the new administration.

The immediate decisions included:

-- Whether to spend money to fly the extra shuttle mission required to ferry the Alpha Magnetic Spectrometer to the orbiting international space station, a priority voiced by Congress.

-- Whether to continue shuttle operations beyond scheduled retirement of the fleet in 2010 at a cost of roughly $3 billion a year for two shuttle flights a year.

-- Whether to extend operations of the orbiting international space station beyond 2015.

-- Whether to speed delivery of the Constellation project by one year to 2015 at an estimated additional cost of nearly $4 billion.

Griffin said Obama's transition team has discussed the possibility of submitting an amended budget to Congress to broadly substitute the priorities of the incoming administration for the outgoing Bush administration.

As required by law, the Bush team has prepared its own version of a federal budget for the fiscal year beginning Sept. 30 that will be handed to Congress in early February. Incoming presidents often amend the budget package filed by predecessors.

Griffin, an engineer, said he would begin looking for a new job after leaving his post next Tuesday.

"All political appointees are required to submit a resignation effective not later than noon on Jan. 20," Griffin said. "That was asked of us in early December. I did that. The new team has my resignation, and as with 3,000 other political appointees, unless they choose to call you and ask you to stay, that resignation becomes effective on the 20th of January."